Process for purging a drill stem

ABSTRACT

A process for purging a drill stem of drilling fluid and relieving the hydrostatic pressure within the stem to perform a production test or introduce treating fluids into a formation at the bottom of the stem. The process comprises the steps of: pumping a liquefied purge gas into the stem to displace the drilling fluid out of the stem and into the annulus; setting an expandable packing carried by the stem; relieving the pressure on the liquefied purge gas; and bleeding the purge gas from the stem. The purge gas is a gas capable of being liquefied at ambient temperatures under moderate pressures yet is gaseous at normal ambient temperatures and atmospheric pressures.

United States Patent {72] Inventors Harold E. Shillander 414% Central S.E.; Roy M. Eidal, P. 0. Box 2087, both of Albuquerque, N. Mex. 8710] [21] Appl. No. 884,287

[22] Filed Dec. 11, 1969 [45] Patented Oct. 12, 197 l [54] PROCESS FOR PURGING A DRILL STEM [56] References Cited UNITED STATES PATENTS 2,663,545 12/1953 Grable 166/264 Primary Examiner-Stephen J Novosad AttorneyFulwider, Patton, Rieber, Lee & Utecht ABSTRACT: A process for purging a drill stem of drilling fluid and relieving the hydrostatic pressure within the stem to perform a production test or introduce treating fluids into a formation at the bottom of the stem. The process comprises the steps of: pumping a liquefied purge gas into the stem to displace the drilling fluid out of the stem and into the annulus; setting an expandable packing carried by the stern; relieving the pressure on the liquefied purge gas; and bleeding the purge gas from the stem. The purge gas is a gas capable of being liquefied at ambient temperatures under moderate pressures yet is gaseous at normal ambient temperatures and atmospheric pressures.

PROCESS FOR PURGING A DRILL STEM BACKGROUND OF THE INVENTION During the course of drilling a well, it is often desirable to test formations at various depths to determine the content of such formations before proceeding further with the drilling operation. Until relatively recent times, it was necessary, each time a test was to be made, to remove the drill stem carried drilling tool from the borehole and replace it with a testing tool for performing the desired formation production test. Combined drilling and testing tools are now available which enable a production test to be performed without requiring that the drilling tool be removed from the borehole. Two such tools are disclosed in U.S. Pat. Nos. 3,233,676 and 3,235,017, each entitled EARTH BOREHOLE DRILLING AND TEST- ING TOOL, isued to J. Lynes on Feb. 8, 1966 and Feb. 15, 1966, respectively. Such combined drilling and testing tools result in substantial savings in both time and labor and reduce the cost of formation testing while simultaneously improving efficiency in overall oil and gas drilling operations.

Generally, as a drilling operation progresses, a drilling fluid is pumped down the inside of the drill stem to remove cuttings produced by the drill bit, the fluid and cuttings being then circulated upwardly through the annulus between the stem and borehole and discharged into a mud pit. The drilling fluid, which is basically water with additives for ensuring that the cuttings are completely picked up and for maintaining the hydrostatic pressure at the bottom of the borehole greater than the pressure of any formation expected to be encountered, is quite heavy and may weigh any where from 9 to well over 12 pounds per gallon.

Fluid weighing l pounds per gallon creates a hydrostatic pressure of approximately 51.9 pounds per square inch for each 100 feet of the borehole depth. Hence, for a borehole 5,000 feet deep, the hydrostatic pressure at the bottom of the hole would be approximately 2,600 pounds per square inch. Due to this relatively high hydrostatic pressure, if a formation is encountered by the drill bit which contains a fluid whose pressure is less than that exerted by the drilling fluid, there will be no way to know that a possibly productive formation has been encountered since the drilling fluid will prevent flow of the gas or oil from the formation in to the borehole. Therefore, in order to avoid continued drilling without knowledge of what is being encountered, it is desirable to selectively stop drilling and test the encountered formations, generally choosing the occasion for such testing by observed changes in drilling penetration rates and examinations of the drilling fluid and cuttings.

In order to perform a production test at any given point in the drilling operation when using a combined drilling and testing tool, for example of the type previously mentioned, it is necessary to remove the drilling fluid from the stem to relieve the hydrostatic pressure so that if the formation does contain pressurized gas or liquid, it will be able to flow upwardly through the stem under its own pressure. There are also other situations wherein it is desirable to remove the drilling fluid and relieve the hydrostatic pressure within the stern for example to gain access through the stem for introducing treating fluids into the formation.

Various methods have been proposed in the prior art for purging the drill stem, one such method being to pump gaseous nitrogen down the stem to displace the drilling fluid therefrom and then removing the nitrogen from the stern. While the use of nitrogen has generally been reasonably satisfactory, numerous difficulties have been encountered.

Since nitrogen is a cryogenic fluid, that is, one which liquifies only at extremely low temperatures, it must be used as a purging fluid in the gaseous state only as its liquid temperature is so low that embrittlcmcnt of the well tools and associated components would result. Being a cryogenic fluid, it must be stored at extremely low temperatures and under relatively high pressures. It is therefor general practice in the industry to have nitrogen supplied to the drill site by a supplier. The supplier, generally an independent contractor who is equipped to handle low-temperature gas in liquid form, supplies nitrogen only when needed, since onsite storage tanks for such cryogenic fluids are not feasible due to the protracted intervals between demands for stem purges.

When the drilling fluid is to be displaced from the stem, the nitrogen, as supplied in liquid form, must be heated to the gaseous state before being pumped down the stem. Thus, complex heat exchangers ai'rd high-pressure pumping apparatus are required for producing the conversion and the cost of such equipment is quite high.

While nitrogen is relatively inexpensive and available in many areas, there are still many areas where nitrogen is not readily available. Therefore if drilling is being performed where nitrogen is not available on immediate demand, the drilling operation will be substantially hindered since each time the stem is to be purged, the entire drilling operation must be shut down until the nitrogen is supplied. Further, due to the compressibility of the gas when subject to high pressures such as those necessary to displace the drilling fluids from the stem, the use of gaseous nitrogen has proved undesirable as it is difficult, if not impossible, to accurately determine how much gas has been pumped into the stem and how much drilling fluid has been displaced into the annulus thereby.

SUMMARY OF THE INVENTION The present invention provides an improved process for purging a drill stem of drilling fluid and relieving the hydrostatic pressure on the bottom of a borehole which enables the drilling rig operator to purge the stem at any time without a prolonged shutdown, and with a substantial reduction in cost. Further, the invention provides a process which allows the purge gas to be stored at the drilling site with inexpensive and readily available present-day equipment, and which enables more precise control of the purging process.

In the preferred form of the present invention illustrated herein, the purging process includes utilizing a liquefied noncryogenic gas held under pressure which is pumped down the stern in liquid form to displace the drilling fluid therefrom, the pressure being sufficient to maintain the purge gas in the liquid state while simultaneously remaining well within the capable pressure limits of the stem. By utilizing a liquid purging process, more precise control of the amount of drilling fluid displaced from the stem can be maintained.

More specifically, as disclosed herein by way of illustration and not necessarily limitation, it is an object of the present invention to provide a process for purging a drill stem of hydrostatic pressure which utilizes a purge gas which is normally gaseous at ambient temperatures and atmospheric pressures, although capable of being liquefied at ambient temperatures by the application of moderate external pressures. It is a further object of the present invention to provide a process wherein the purge gas may be used in the liquid state during displacing of the drilling fluid and may be readily stored for long periods of time at the drill site in presently available, conventional pressure tanks, tubes or other storage vessels.

The many features and advantages of the present invention will be pointed out in the following detailed description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principles of the invention and the best mode which has been contemplated by applying these principles.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the beginning of the novel process of the subject invention wherein the liquefied purge gas is being pumped into the top of the drill stem under pressure;

FIG. 2 illustrates a further phase in the novel process wherein the liquefied purge gas has substantially completely displaced the drilling fluid from the drill stem; and

FIG. 3 illustrates the completion of the purging process wherein the internal pressure on the drill stem is being relieved.

DETAILED DESCRIPTION As shown in the drawings for the purposes of illustration, the present invention relates to an improved process for relieving hydrostatic pressure within a drill stem 10 in situ within a borehole 12. In this instance, the borehole is formed by a combined drilling and testing tool of the general type disclosed in the aforesaid patents, the stem carrying a drill bit 14 on its lower end and an expandable annular packing sleeve 16 spaced above the bit.

In order to pick up cuttings formed by the bit 14, and to maintain the hydrostatic pressure at the bottom of the borehole 12 greater than the pressure of any fluid in any formation expected to be encountered during drilling, a drilling mud or fluid 18 is continuously circulated through the stem 10 and borehole. The drilling fluid is pumped downwardly through the stem to the bit where it picks up the cuttings and carries them upwardly through an annular space 20 between the borehole and the stem, to a discharge pipe 22 and a mud pit 24, as illustrated by the arrows 26 of FIG. 1.

During drilling, the packing 16 is retracted to allow the drilling fluid 18 to circulate freely through the annular space 20 as it moves upwardly from the bit 14. The packing may be of any suitable type which can be selectively activated to the expanded position (see FIG. 3) whereby the portion of the annular space above the packing is sealed from that below.

When it is desirable to gain access through the stem 10 to a formation at the bottom of the borehole 12, for example to perform a production test or to introduce treating fluids into the formation, it is necessary to purge the stern by removing the drilling fluid l8 and relieving the hydrostatic pressure at the bottom of the borehole.

In accordance with the process of the present invention, the hydrostatic pressure at the bottom of the borehole 12 is relieved by the steps of pumping a preselected volume of a liquefied purge gas 28 down the stem 10 under sufficient pressure to maintain the liquefied gas in the liquid state and to displace the drilling fluid 18 from the stem, setting the packing 16 after displacing substantially all of the drilling fluid from the stern, relieving the pressure on the liquefied gas and bleeding the gas from the stem. The purge gas has the characteristics of being gaseous at normal atmospheric pressures and ambient temperatures, and is capable of liquefying under moderate pressured at ambient temperatures.

In this instance, the purge gas 28 is stored at the drill site in a suitable storage tank 30, preferably a conventional pressure container adapted to store liquefied gas at ambient temperatures, for example between and 120 F., under moderate pressured sufficient to maintain the gas in the liquid state, typically less than 5,000 pounds per square inch (p.s.i.). The storage tank can be connected to the upper end of the stem in any suitable manner, and the liquefied purge gas 28 is pumped into the stem by a high-pressure pump 32, herein operated by a motor 34 coupled in a pressure line 36 between the tank and stem.

As illustrated in FIG. 1, the stem 10 and the annular space are filled with drilling fluid 18 after halting the rotation of the bit 14 and the drilling fluid circulating pump (not shown). A flow regulator valve 38 in the drilling fluid discharge pipe 22 controls the flow of drilling fluid from the annular space into the mud pit 24, and is adjustable to produce a selected back pressure or flow resistance in the drilling fluid as it is displaced out of the stem.

Herein, a two-position flow valve 40 in the pressure line 36 interconnects the stem 10 and tank 28 in one position, as seen in FIGS. 1 and 2, and in the second position, shown in FIG. 3, interconnects the stem with a bleed pipe 42. After halting the drilling operation and connecting the pressure line to the The motor-driven plump 32 supplies the liquefied purge gas 28 from the tank 30 to the stem 10, and applies sufficient pressure on the liquefied gas to displace the drilling fluid l8 downwardly through the stem 10 and upwardly through the annular space 20 and regulator valve 38 to the mud pit 24. As a result of the controlled back pressure created by the regulator valve, the pressure necessary to displace the drilling fluid can be selected by properly setting the regulator valve, so that pressure will also be sufficient to maintain the purge gas in the liquid state during displacement. Since the purge gas is maintained in the liquid state during the displacing operation, the volume of drilling fluid removed from the stem will substantially equal the volume of purge liquid pumped therein, liquid being virtually incompressible.

When purging the stem 10 for performing a production test or to introduce treating fluids, it is generally undesirable to have the purge gas 28 enter the annular space 20, as this will result in aeration of the drilling fluid 18 in the annular space and produce gas kicks. Therefore, precise control of the amount of drilling fluid displaced from the stem must be maintained.

By virtue of using a liquefied purge gas, the precise volume of purge gas needed for substantially completely removing the drilling fluid from the stern can be readily determined by knowing the depth of the borehole and the size and capacity of the stem. Thus, only that volume necessary to remove the drilling fluid is pumped down the stem, thereby virtually eliminating the possibility of gas kicks.

In one presently preferred example, propane has been found to be highly satisfactory for use as the purge gas. Propane, although flammable when mixed with oxygen, can readily be used with the process of the present invention with complete safety, as it can be maintained in the liquid state by a pressure of 124 pounds per square inch at an ambient temperature of 70 F., and will not react with substances likely to be encountered during drilling.

Propane is a comparatively inexpensive gas and is readily available in most places. Further, with the relatively low pressures necessary to maintain it in the liquid state at ambient temperatures, it may be readily stored in conventional pressurized storage tanks for long periods of time at the drilling site.

Other examples of gases which have been found to be suitable for use in the process of the present invention are butane, carbon dioxide and natural gas. Each of these gases are readily maintained n the liquid state under moderate pressured, for example less than 5,000 p.s.i., at ambient temperatures, and are normally gaseous at atmospheric pressures and ambient temperatures. These gases when in the liquid state, like propane, are well within the temperature and pressure limitations of presently available drilling equipment and may be used with complete safety. These are readily available in most areas and are inexpensive in cost, and they can be stored for long periods of time in presently available, moderately priced pressure vessels.

Referring now to FIG. 2, after displacing the desired volume of drilling fluid, 18 from the inside of the stem 10, the stem is filled with the liquefied purge gas 28, the pressure of this liquefied gas at the bottom of the borehole 14 being substantially equal to that of the drilling fluid which previously occupied the stem. At this point, the annular space 20 is full of drilling fluid which functions to support the wall of the borehole and restrain it against a cave-in around the stem during the subsequent test or treatment operations.

After completion of the displacing operation, as illustrated in FIG. 3, the expandable packing I6 is set to seal off the annular space 20 above the packing from that below, thus holding the drilling fluid 18 above the bottom of the borehole 12. The flow valve 40 in the pressure line 36 between the top of stem 10 and the pump 32 is then positioned to close ofi the pump and connect the top of the stem with the bleed pipe 42, which preferably vents to atmosphere. As a result of communication of the top of the stem with the bleed pipe 42, the pres sure on the liquefied purge gas is relieved and the purge gas can undergo a change of state from the liquid state to the gaseous state.

As the pressure within the stem drops, the purge gas 28 changes state and expands upwardly and out of the stem, thus substantially relieving the internal pressure within the stem and on the bottom of the borehole 12. To aid in the internal pressure relief, a suction pump (not shown) can be provided for helping remove the purge gas from the stem.

After relieving the hydrostatic pressure within the stem 10 in this way, the stem will be free of drilling fluid l8 and the formation at the bottom of the borehole 12 will be readily accessible through the stem. Thus the production test or the introduction of treating fluids can be performed as desired.

In performing a production test, after relieving the hydrostatic pressure on the bottom of the borehole 12 in the above manner, should the formation encountered by the bit 14 at this point contain a pressurized fluid, such as gas or oil, the internal pressure of this substance may be sufficient to force it upwardly through the stem 10, as illustrated by the arrows 44 of FIG. 3, and subsequently outwardly through the bleed pipe 42, whereupon it will be detected.

If, however, there is no flow from the formation to the surface, this would indicate that either the formation does not contain pressurized gas or oil or that if it does contain gas or oil, the pressure thereof is insufficient to lift the contents to the surface. In order to ascertain which of the two possibilities exist, it is simply necessary to release the packing l6 and reverse flow the drilling fluid 18 downwardly through the annular space 20 and then upwardly through the stem. As the drilling fluid is pumped upwardly through the stem, the contents of the stem will then come to the surface for observation and measurement, having been pushed upwardly by the drilling fluid.

Thus, the present process enables the drilling rig operator, when using a combined drilling and testing tool, to terminate drilling at any desired point and immediately perform a production test or introduce treating fluids to the formation, without the necessity of encountering time delays through lack of availability of purge gas. Further, the purging process of the present invention is both effective in operation and inexpensive in cost.

While the particular discussion above has been directed to the preferred embodiment of this invention, it is apparent that modifications and variations therein may be made without departing from the spirit and scope of the novel concepts of this invention.

We claim:

1. A process for displacing drilling fluid from the inside of a drill stem having an open lower end into an annular space between said stem and a borehole, and relieving internal pressure within said stem, said process comprising the steps of:

displacing substantially all drilling fluid from said stem by pumping a preselected volume of liquified gas into said stem substantially equal to the internal volume of said stem and under sufficient pressure to maintain said gas in the liquid state, thereby to displace said drilling fluid out of said stem and into said annular space while leaving said lower end open;

temporarily sealing said annular space adjacent said lower end;

and relieving said pressure after sealing said annular space thereby to allow said liquefied gas to change to the gaseous state and bleed from said stem.

2. A process as in claim 1 wherein said gas is propane.

3. A process as in claim 1 wherein said gas is butane.

4. A process as in claim 1 wherein said gas is natural gas.

5. A process as in claim 1 wherein said gas is carbon dioxatmospheres at a temperature within the range of 0 to l20 F.

8. A process of displacing drilling fluid from a drill stern into an annular space between said stem and a borehole and relieving internal pressure from within said stem for performing a formation production test, said process comprising the steps of:

pumping a preselected volume of liquified gas into said stem under sufficient pressure to maintain said gas in the liquid state within said stem whereby said drilling fluid is displaced from said stem;

sealing said annular space adjacent the bottom of said stem after completion of said pumping;

relieving said pressure after sealing said annular space thereby to allow said liquefied gas to change to the gaseous state; and

directing said gas out of said stern whereby said stem is relieved of internal pressure and said production test may be performed.

9. A process as set forth in claim 8 wherein said gas is of the type capable of liquefying under a pressure of less than 5,000 pounds per square inch at ambient temperatures.

10. A process as set forth in claim 9, further including the step of providing a supply of said gas adjacent said borehole.

11. A process for displacing drilling fluid from a drill stem into an annular space between said stem and a borehole and relieving internal pressure within said stem to perform a production test wherein said stem is of the type including a combined drilling and testing tool having an expandable packing adjacent the drilling end of said stem, said process comprising the steps of:

pumping a liquefied gas into said stern under sufficient pressure to displace said drilling fluid and to maintain said gas in the liquid state within said stem;

setting said expandable packing after completion of said pumping thereby to seal said annular space;

relieving said pressure after setting said packing thereby to allow said liquefied gas to change to the gaseous state;

and bleeding said gas from said stem whereby said stem is relieved of internal pressure and said production test may be performed.

12. A process as set forth in claim 11 wherein a volume of liquefied gas substantially equal to the internal volume of said stem is pumped into said stem whereby substantially all of said drilling fluid is displaced from said stem into said annular space.

13. A process as set forth in claim 12 wherein said setting of said packing holds said drilling fluid in said annular space above said drilling end of said stem.

14. A process as set forth in claim 13 further including the step of releasing said packing after performing said test whereby said drilling fluid reenters said stem from said annular space.

15. A process as set forth in claim 11 further including the step of providing a supply of liquefied gas adjacent said borehole, said gas being of the type capable of liquefying under a pressure less than 5,000 pounds per square inch at ambient temperatures and which is normally gaseous at atmospheric pressures and ambient temperatures.

16. A process as set forth in claim 15 wherein said gas is selected from the group consisting of propane, butane, carbon dioxide and natural gas. 

1. A process for displacing drilling fluid from the inside of a drill stem having an open lower end into an annular space between said stem and a borehole, and relieving internal pressure within said stem, said process comprising the steps of: displacing substantially all drilling fluid from said stem by pumping a preselected volume of liquified gas into said stem substantially equal to the internal volume of said stem and under sufficient pressure to maintain said gas in the liquid state, thereby to displace said drilling fluid out of said stem and into said annular space while leaving said lower end open; temporarily sealing said annular space adjacent said lower end; and relieving said pressure after sealing said annular space thereby to allow said liquefied gas to change to the gaseous state and bleed from said stem.
 2. A process as in claim 1 wherein said gas is propane.
 3. A process as in claim 1 wherein said gas is butane.
 4. A process as in claim 1 wherein said gas is natural gas.
 5. A process as in claim 1 wherein said gas is carbon dioxide.
 6. A process as in claim 1 wherein said gas is selected from a group consisting of propane, butane, natural gas and carbon dioxide.
 7. A process in claim 1 wherein said gas is of the type capable of liquefying under a pressure within the range of 2 to 350 atmospheres at a temperature within the range of 0* to 120* F.
 8. A process of displacing drilling fluid from a drill stem into an annular space between said stem and a borehole and relieving internal pressure from within said stem for performing a formation production test, said process comprising the steps of: pumping a preselected volume of liquified gas into said stem under sufficient pressure to maintain said gas in the liquid state within said stem whereby said drilling fluid is displaced from said stem; sealing said annular space adjacent the bottom of said stem after completion of said pumping; relieving said pressure after sealing said annular space thereby to allow said liquefied gas to change to the gaseous state; and directing said gas out of said stem whereby said stem is relieved of internal pressure and said production test may be performed.
 9. A process as set forth in claim 8 wherein said gas is of the type capable of liquefying under a pressure of less than 5,000 pounds per square inch at ambient temperatures.
 10. A process as set forth in claim 9, further including the step of providing a supply of said gas adjacent said borehole.
 11. A process for displacing drilling fluid from a drill stem into an annular space between said stem and a borehole and relieving internal pressure within said stem to perform a production test wherein said stem is of the type including a combined drilling and testing tool having an expandable packing adjacent the drilling end of said stem, said process comprising the steps of: pumping a liquefied gas into said stem under sufficient pressure to displace said drilling fluid and to maintain said gas in the liquid state within said stem; setting said expandable packing after completion of said pumping thereby to seal said annular space; relieving said pressure after setting said packing thereby to allow said liquefied gas to change to the gaseous state; and bleeding said gas from said stem whereby sAid stem is relieved of internal pressure and said production test may be performed.
 12. A process as set forth in claim 11 wherein a volume of liquefied gas substantially equal to the internal volume of said stem is pumped into said stem whereby substantially all of said drilling fluid is displaced from said stem into said annular space.
 13. A process as set forth in claim 12 wherein said setting of said packing holds said drilling fluid in said annular space above said drilling end of said stem.
 14. A process as set forth in claim 13 further including the step of releasing said packing after performing said test whereby said drilling fluid reenters said stem from said annular space.
 15. A process as set forth in claim 11 further including the step of providing a supply of liquefied gas adjacent said borehole, said gas being of the type capable of liquefying under a pressure less than 5,000 pounds per square inch at ambient temperatures and which is normally gaseous at atmospheric pressures and ambient temperatures.
 16. A process as set forth in claim 15 wherein said gas is selected from the group consisting of propane, butane, carbon dioxide and natural gas. 